squeezing the bug

We had an insidious bug popping up from time to time that was affecting the player’s car and sometime enemies too. This kind of bug are very hard to fix because comes out totally random with no apparently reason. It was a lucky strike when I discovered something interesting about it.
I’ve set up a test scene with the player’s car instanced twice.
I’ve saved the car scene in scn format and in tscn format, so, one instance was made from scn while the other form tscn.
It happens that the bug affects only the scn car.
Here a screen-capture on some test. Both cars are contolled by the player, the left car is the tscn instance while the right car is the scn instance, as you can see the car on the right is randomly affected from the bug.

edit:

further test revealed that is not the scene format but the position in the scene tree, I mean, the first car in the scene tree is affected by the bug while the second is not